The “mixed raster content” (MRC) representation of documents provides the ability to represent color images and either color or monochrome text. In an MRC representation, an image is represented by more than one image plane. The main advantage of the MRC representation of documents is to provide an efficient way to store, transmit, and manipulate large digital color documents. The method exploits the properties of the human vision system, where the ability to distinguish small color variations is greatly reduced in the presence of high-contrast edges. The edge information is normally separated from the smoothly varying color information, and encoded in one of the planes, called the mask plane. Following a careful separation, the various planes can be independently compressed using standard compression schemes (such as JPEG and G4) with good compression and high quality at the same time.
The general MRC representation typically comprises three independent planes: foreground, background, and a mask plane. The background plane is typically used for storing continuous-tone information such as smoothly varying background colors. The mask plane normally holds the image of text (binary) as well as other edge information (e.g., line art drawings). The foreground plane usually holds the color of the corresponding text and/or line art. However, the MRC representation specifies only the planes and their associated compression methods: it does not otherwise restrict nor enforce the content of each of the planes. The content of each of the planes may be defined appropriately by an implementation of the MRC representation.
The foreground and background planes are defined to be two full-color (L, a, b) or YCC planes. The mask plane is defined as a binary (1-bit deep) plane. One exemplary MRC representation specifies that the foreground and background are to be JPEG compressed, or more generally, can be a suitable image for a lossy compression technique, and that the mask plane is to be ITU-G4 compressed, or more broadly is suitable for a lossless compression technique. In general, the foreground, background, and mask planes can all have different spatial resolutions, and they are not required to maintain the original source input resolution.
In a practical implementation of scanning image data and rendering it in MRC, there are two basic challenges that impede the efficiency of processing of the image data. One, in a large scanning job there is often a mix of single-side imaged and two-side imaged sheets: it is most efficient to simply scan both sides of every sheet, but this will leave the unused sides of single-sided sheets to be recorded and stored as meaningless data. Two, black-text-only images submitted to MRC representation result in a mask plane and a background plane, but the background plane represents redundant data; however, it is impractical to pre-separate text-only originals from the rest of a set of images. Photograph-only images yield the opposite situation: in such a case the mask plane would be redundant.
Another challenge to efficient MRC compression is presented by document images that include “highlights”, “watermarks”, or similar areas of light (near-white in luminance) and uniform coloration. Such areas occur in an image when, for example, digital or physical watermarks or graphics are present in the original image, or if the original image comprises text printed on lightly-colored paper. These images present opportunities to realize efficiencies in compression.
In some cases, the MRC processing can be designed to provide only background and mask planes without a foreground plane. The mask plane could represent black text while the background representing the smooth transitioned (such as photograph) area. This type of MRC representation is often called two-layer MRC representation. The MRC representation can also comprise one background layer and multiple mask layers: the type of representation is generally called multiple layer MRC or multiple mask MRC. For illustration purposes, the embodiment is explained using two-layer MRC representation, although embodiments based on three or more layers are possible.
U.S. Pat. No. 5,854,857 discloses an enhancement to the standard JPEG compression technique, including a step of recording the length of each string of bits corresponding to each block of pixels in the original image at the time of compression. The list of lengths of each string of bits in the compressed image data is retained as an “encoding cost map” or ECM. The ECM, which is considerably smaller than the compressed image data, can be transmitted or retained in memory separate from the compressed image data along with some other accompanying information and used as a “key” for editing or segmentation of the compressed image data.
U.S. Patent Application Publication 2002/0076103 A1 gives a general overview of an MRC-based compression technique.
U.S. Patent Application Publication 2003/0072487 A1 discloses an image segmentation technique which identifies a main background and one or more objects in an image being processed. Objects that cannot be classified as text or pictures are further segmented into a local background and smaller objects.